The aim of biomedical research is to gain a better understanding of human physiology and to use this knowledge to prevent, treat or cure human diseases. Due to practical and ethical barriers to the experimentation on human subjects, many studies are conducted on small animal models, such as the mouse. However, mice are not people and the knowledge gained from animal experimentation is not always applicable to humans. In this context, mice repopulated with a human hemato-lymphoid system (HHLS) represent a useful small animal model for the study of human hematopoiesis and immune function in vivo.
HHLS mice are generated by the transplantation of human hematopoietic stem and progenitor cells (HSPCs) and/or human fetal tissues into recipient mice deficient in the innate and adaptive arms of the immune response. The first models of HHLS mice were developed in the late 1980s (Mosier et al., 1988, Nature 335:256-259; McCune et al., 1988, Science 241:1632-1639; Kamel-Reid and Dick, 1988, Science 242:1706-1709), and have been undergoing a series of improvements since then (Legrand et al., 2006, Journal of Immunology 176:2053-2058; Shultz et al., 2007, Nature Reviews Immunology 7:118-130). The strains of mice currently used as recipients for human hematopoietic engraftment share three characteristics. First, they lack B and T cells due to the Scid mutation in the gene encoding the PRKDC protein (Mosier et al., 1988, Nature 335:256-259; McCune et al., 1988, Science 241:1632-1639), or due to deletion of one of the two Rag genes (Shultz et al., 2000, Journal of immunology 164:2496-2507; Traggiai et al., 2004, Science 304:104-107). Second, deletion or mutation of the Il2rg gene that encodes the common gamma chain (γc) of cytokine receptors abolishes IL-15 signaling and results in the absence of NK cells (Traggiai et al., 2004, Science 304:104-107; Ito et al. 2002, Blood 100:3175-3182). Third, the interaction between the SIRPA receptor expressed on mouse macrophages and the CD47 ligand on human cells provides an inhibitory signal to mouse macrophages and confers phagocytic tolerance for the human xenograft (Takenaka et al., 2007, Nature Immunology 8:1313-1323; Takizawa & Manz, 2007, Nature Immunology 8:1287-1289). Cross-species interaction between SIRPA expressed on mouse cells and human CD47 is achieved when using the NOD genetic background which contains a natural polymorphism in the Sirpa gene (Takenaka et al., 2007, Nature Immunology 8:1313-1323; Takizawa & Manz, 2007, Nature Immunology 8:1287-1289; Legrand et al., 2011, Proc Natl Acad Sci USA 108:13224-13229) or by BAC-transgenic expression of the human SIRPA gene (Strowig et al., 2011, Proc Natl Acad Sci USA 108:13218-13223). High levels of human hematopoietic cell engraftment, upon human HSPC transplantation, are achieved when using NOD Scid γc−/− (NOG (Ito et al. 2002, Blood 100:3175-3182) or NSG (Ishikawa et al., 2005, Blood 106:1565-1573)) or hSIRPAtg RAG2−/−γc−/− (SRG (Strowig et al., 2011, Proc Natl Acad Sci USA 108:13218-13223)) mice as recipients.
Although human multi-lineage hematopoietic development is observed in these recipient strains, the terminal differentiation, homeostasis and/or effector function of most human cell types is sub-optimal. It has been hypothesized that this condition is due to reduced or absent cross-reactivity between cytokines secreted by mouse tissues and the human receptors expressed on hematopoietic cells (Manz, 2007, Immunity 26:537-541; Willinger et al., 2011, Trends in Immunology 32:321-327). To circumvent this limitation, several strategies have been developed to deliver human cytokines in the mouse host. These methods include the injection of recombinant cytokines (Lapidot et al., 1992, Science 255:1137-1141; van Lent et al., 2009, J. Immunol 183:7645-7655), lentiviral delivery of cytokine-encoding cDNA (O'Connell et al., 2010, PloS One 5(8):e12009), hydrodynamic injection of plasmid DNA (Chen et al., 2009, Proc Natl Acad Sci USA 106:21783-21788), transgenic expression of cDNA (Nicolini et al., et al., 2004, Leukemia 18(2):341-347; Brehm et al., 2012, Blood 119:2778-2788; Takagi et al., 2012, Blood 119:2768-2777) or knock-in replacement of cytokine-encoding genes (Rongvaux et al., 2011, Proc Natl Acad Sci USA 108:2378-2383; Willinger et al., 2011, Proc Natl Acad Sci USA 108:2390-2395; Rathinam et al., 2011, Blood 118:3119-3128). The later method has the advantage of more physiological expression of the human gene. Furthermore, if the human cytokine is not fully cross-reactive on the mouse receptor, it can induce a defect in mouse cell populations and confer an additional competitive advantage to human cells. Using a knock-in gene replacement strategy, humanization of the gene encoding thrombopoietin (Tpo) resulted in better maintenance of functional human hematopoietic stem cells and increased engraftment in the bone marrow (Rongvaux et al., 2011, Proc Natl Acad Sci USA 108:2378-2383); replacement of the genes encoding interleukin-3 and GM-CSF (Il3 and Csf2) induced the loss of mouse lung alveolar macrophages (AM) and the development of functional human AM (Willinger et al., 2011, Proc Natl Acad Sci USA 108:2390-2395); and substitution of the Csf1 gene, which encodes M-CSF, resulted in increased numbers of human monocytes in multiple tissues (Rathinam et al., 2011, Blood 118:3119-3128).
Human and mouse hemato-lymphoid systems differ in many aspects (Haley, 2003, Toxicology 188:49-71; Mestas & Hughes, 2004, J Immunol 172:2731-2738). One of the major differences between the two species lies in their white blood cell (WBC) differential. Human blood is rich in myeloid cells that represent 50-75% of total WBCs. In contrast, mouse blood is dominated by lymphocytes and only 20-30% of WBCs are of myeloid lineages. This species difference, whose functional and evolutionary significance is not understood, is not recapitulated in conventional HHLS mice such as NOG/NSG or SRG. Indeed, human myeloid development is particularly defective in these hosts, with myeloid cells representing only 5-10% of human WBCs.
One application of mice with functional human immune systems is the development and testing of human vaccines. Historically, the induction of immune responses in vivo has been relatively inefficient (2004, Traggiai et al., Science 304:104-107; 2002, Ito et al., Blood 100:3175-3182; 2005, Ishikawa et al., Blood 106:1565-1573; 2005, Shultz et al., J Immunol 174:6477-6489; 2006, Baenziger et al., Proc Natl Acad Sci USA 103:15951-15956). Several studies have reported successful pathogen-specific immune responses upon infection. Although it was reported that around 50% of mice produced virus-specific IgM and IgG upon dengue virus infection (2007, Kuruvilla et al. Virology 369:143-152), other studies reported frequencies below 20% of mice producing antigen-specific IgM and IgG after HIV and EBV infection (2006, Baenziger et al., Proc Natl Acad Sci USA 103:15951-15956; 2008, Yajima et al., J Infect Dis 198:673-682). Upon immunization with adjuvant and antigen, class switching of antigen-specific immunoglobulins is also historically inefficient with only a fraction of immunized animals showing antigen specific IgG responses (2004, Traggiai et al., Science 304:104-107; 2002, Ito et al., Blood 100:3175-3182; 2005, Ishikawa et al., Blood 106:1565-1573; 2005, Shultz et al., J Immunol 174:6477-6489; 2009, Watanabe et al., Int Immunol 21:843-858; 2010, Becker et al., PLoS ONE 5). These studies included NSG and BALB/c RAG2−/− γc−/− mice and different adjuvant/antigen combinations.
There is a need in the art for humanized non-human animals able to support and sustain engraftment with human hematopoietic cells. The present invention addresses this unmet need in the art.